In many polymerization processes for the production of polymers of .alpha.-monoolefins, a polymerization stream is formed which is a slurry of particulate polymer solids suspended in a liquid reaction diluent. Exemplary of such processes are those disclosed in U.S. Pat. Nos. 3,172,737 (Whittington, Mar. 9, 1965), 3,242,150 (Scoggin, Mar. 22, 1966), 3,262,922 (Payne, July 26, 1966), 3,318,857 (Dietz, May 9, 1967), 4,121,029 (Irvin and Sherk, Oct. 17, 1978), and 4,424,341 (Hanson and Sherk, Jan. 3, 1984).
A particularly suitable method and apparatus for polymerization of .alpha.-monoolefins involves an elongated tubular closed loop (endless loop) reaction zone, such as described in the above listed patents. In such polymerization modes, a catalyst, hydrocarbon diluent, and monomer(s) are added generally continuously to and are moved continuously through a relatively smooth-path endless loop at fluid velocities in the highly turbulent range (such as about 10 to 25 ft per sec). The monomer(s) polymerize and precipitate as a slurry of solid form particulate polymer suspended in the diluent.
In a commercial scale operations, it is desirable to separate the particulate polymer from the diluent in the recovery step in a manner such that the diluent is not exposed to contamination so as to permit recycle of the diluent to the polymerization zone with minimal if any purification.
Typically, in the particle-form polymerization for polyethylene, the composition of the slurry in the reactor is of the order of, by weight, particulate polymer about 30 percent, heat transfer liquid composed of diluent about 68 percent, and monomer about 2 percent, though these proportions can vary considerably.
Separating the particulate polymer from the diluent typically has involved the use of a vertical settling leg or dump leg such that the flow of slurry across the opening thereof provides a zone where the polymer particles can settle to some extent from the diluent. The accumulated slurry of polymer particles and diluent, collected in one or more vertical settling legs, is discharged periodically to a flash chamber wherein the mixture is flashed for substantially complete removal of diluent and any traces of monomer, such as disclosed in U.S. Pat. No. 3,152,872 (Scoggin et al, Oct. 13, 1964). The diluent and any monomer vapors are recovered, condensed, and reused in the polymerization process.
Settling legs have been located variously around the reactor loop, in efforts to obtain efficient settling of the particulate polymers so that the polymer particles can be recovered and the diluent recycled as expeditiously and economically as possible. Nevertheless, settling efficiencies have remained lower than desired.
Heretofore, all of the diluent and monomer in the slurry ultimately has had to be flashed in order to separate the solid particulate polymer.
If sizable reductions in diluent, and corresponding increases in polymer content, could be achieved in the material collected in the settling legs, such would mean sizable increases in reactor efficiencies since significantly less diluent would be subject to flashing, and the consequent necessities of cooling, compression, and condensation for recycle.